1. Field of the Invention
The following disclosure and the appended claims are directed to a lens for distributing light from a plurality of linearly arranged point light sources.
2. Description of the Related Art
Light emitting diodes (LEDs) are rapidly entering the general illumination market, because of their ever-decreasing prices and ever-increasing luminous efficacy, as well as their compactness, ruggedness, and long operating life. The expanding market for LEDs as illumination sources will generate enormous national energy savings, as well as significant waste reduction from the elimination of short-lived and relatively bulky light-bulb discards. The compactness of LEDs enables precision plastic optics to be economically manufactured and integrated into lighting modules tailored for particular illumination tasks.
A prominent lighting task that is poorly served without such tailoring is shelf lighting. In the case of a line of un-lensed small light sources, shelf lighting is necessarily uneven, with the illuminance falling off greatly away from the light source. With LEDs, it is possible to use lensing that will redistribute light in order to produce uniform illumination across a shelf. The two major types of lensing are individual lensing and array lensing. Individual lensing means that each LED can has a respective lens that distributes light from that LED only. Array lensing means that a line of LEDs has a cylindrically symmetric extruded lens, also known as a linear lens, for illuminating a length of shelving. Array lensing is economically advantageous because a single extruded lens replaces numerous individually molded lenses. Thus it is much easier to mount the single lens over a circuit board having a line of LEDs. For example, instead of having 50 LEDs and 50 lenses for an array of LEDs, only three parts need to be manufactured. A long circuit-board, for the array of LEDs is mounted on an extruded metal railing (or base), and an extruded plastic lens is mounted on the railing above the circuit board.
The use of a single lens for an array of LEDs presents problems. For example, it is relatively difficult to precisely extrude a lens having a thick cross section due to the uneven flow and cooling exhibited by the thicker cross sections. Accordingly, in lighting systems where the light must be bent over a large angle, it is advantageous to reduce lens-thickness by utilizing Fresnel facets. Fresnel facets eliminate the lens thickness required for smooth surfaces by providing the requisite local surface slopes for the desired refractive deflection. Conventional linear Fresnel lenses are imaging lenses and have shapes designed to minimize aberrations. For example, such conventional lenses are frequently used for solar concentration. Typically, such solar concentrators are track on a polar axis so that the sun is never more out of plane than the 23 degrees of solstice, which only causes a minimal focal blurring via reduction in focal length. Linear Fresnel lenses for solar concentration generally do not have to handle out-of-plane rays and are not useful for handling light that impinges on the lenses at substantial angles, such as occurs in a linear array of LEDs used for illumination. To date, no linear Fresnel lenses are available for illumination of nearby planar targets from a linear array of LEDs.